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/* SPDX-License-Identifier: GPL-2.0-only */
#ifndef _LINUX_RANDOMIZE_KSTACK_H
#define _LINUX_RANDOMIZE_KSTACK_H
#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
#include <linux/kernel.h>
#include <linux/jump_label.h>
#include <linux/percpu-defs.h>
DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
randomize_kstack_offset);
DECLARE_PER_CPU(u32, kstack_offset);
/*
* Do not use this anywhere else in the kernel. This is used here because
* it provides an arch-agnostic way to grow the stack with correct
* alignment. Also, since this use is being explicitly masked to a max of
* 10 bits, stack-clash style attacks are unlikely. For more details see
* "VLAs" in Documentation/process/deprecated.rst
*
* The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
* only with Clang and not GCC). Initializing the unused area on each syscall
* entry is expensive, and generating an implicit call to memset() may also be
* problematic (such as in noinstr functions). Therefore, if the compiler
* supports it (which it should if it initializes allocas), always use the
* "uninitialized" variant of the builtin.
*/
#if __has_builtin(__builtin_alloca_uninitialized)
#define __kstack_alloca __builtin_alloca_uninitialized
#else
#define __kstack_alloca __builtin_alloca
#endif
/*
* Use, at most, 10 bits of entropy. We explicitly cap this to keep the
* "VLA" from being unbounded (see above). 10 bits leaves enough room for
* per-arch offset masks to reduce entropy (by removing higher bits, since
* high entropy may overly constrain usable stack space), and for
* compiler/arch-specific stack alignment to remove the lower bits.
*/
#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
/**
* add_random_kstack_offset - Increase stack utilization by previously
* chosen random offset
*
* This should be used in the syscall entry path when interrupts and
* preempt are disabled, and after user registers have been stored to
* the stack. For testing the resulting entropy, please see:
* tools/testing/selftests/lkdtm/stack-entropy.sh
*/
#define add_random_kstack_offset() do { \
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \
u32 offset = raw_cpu_read(kstack_offset); \
u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
/* Keep allocation even after "ptr" loses scope. */ \
asm volatile("" :: "r"(ptr) : "memory"); \
} \
} while (0)
/**
* choose_random_kstack_offset - Choose the random offset for the next
* add_random_kstack_offset()
*
* This should only be used during syscall exit when interrupts and
* preempt are disabled. This position in the syscall flow is done to
* frustrate attacks from userspace attempting to learn the next offset:
* - Maximize the timing uncertainty visible from userspace: if the
* offset is chosen at syscall entry, userspace has much more control
* over the timing between choosing offsets. "How long will we be in
* kernel mode?" tends to be more difficult to predict than "how long
* will we be in user mode?"
* - Reduce the lifetime of the new offset sitting in memory during
* kernel mode execution. Exposure of "thread-local" memory content
* (e.g. current, percpu, etc) tends to be easier than arbitrary
* location memory exposure.
*/
#define choose_random_kstack_offset(rand) do { \
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \
u32 offset = raw_cpu_read(kstack_offset); \
offset ^= (rand); \
raw_cpu_write(kstack_offset, offset); \
} \
} while (0)
#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
#define add_random_kstack_offset() do { } while (0)
#define choose_random_kstack_offset(rand) do { } while (0)
#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
#endif
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